We used these for model making in college. You can use a CAD drawing of a product to get a rough model, depending on the quality. It’s amazingly cheaper than a CNC machine and can produce a good result. Some of the ones I worked with used either a type of wax or a dry “glue” to produce the models.

The “dry glue” machine was interesting to watch. It essentially had a pan of very fine powder (looked like a kitty litter tray) then the print head would make pass after pass as a thin layer of the powder was spread across the top after each pass. The tray would slowly lower itself a tiny bit after each pass and you had to dig out the model when it was done. This didn’t produce a perfectly accurate result as the effect of bleeding would attract a little more of the material than you wanted. It was impossible to get a sharp corner but a little sanding could fix that. Still easier and more accurate than carving the entire thing from a block of balsa foam.

Parallelus said
We used these for model making in college. You can use a CAD drawing of a product to get a rough model, depending on the quality. It’s amazingly cheaper than a CNC machine and can produce a good result. Some of the ones I worked with used either a type of wax or a dry “glue” to produce the models.
The “dry glue” machine was interesting to watch. It essentially had a pan of very fine powder (looked like a kitty litter tray) then the print head would make pass after pass as a thin layer of the powder was spread across the top after each pass. The tray would slowly lower itself a tiny bit after each pass and you had to dig out the model when it was done. This didn’t produce a perfectly accurate result as the effect of bleeding would attract a little more of the material than you wanted. It was impossible to get a sharp corner but a little sanding could fix that. Still easier and more accurate than carving the entire thing from a block of balsa foam.

We’re currently printing the parts for the 2nd and 3rd RepRap printer. You can’t print the entire printer again, still have to buy some of the metal parts, but each “revision” of the printer has more and more printed parts than the last, so one day you will be able to “replicate” most of the printer from another printer. Pretty cool!

Here is a website that you can upload your own 3D designs (or buy other designs) and get them printed then delivered to your door: http://www.shapeways.com/

Here is the database of things that can be printed on a RepRap 3D printer: http://www.thingiverse.com/ this is a great website because you can see the quality that you are likely to get out of a cheap printer that is built yourself (like the reprap).